Atrial natriuretic peptide (ANP) is a recently discovered hormone with diverse physiologic properties. Studies are elucidating the role of ANP in multiple pathologic conditions, but its exact importance remains unclear. The gene for mouse ANP has been cloned and sequenced in the sponsor's laboratory. The goal of the Phase I project will be to inactivate the ANP gene in mice. Specifically, the aims are to 1) Construct embryonic stem (ES) cells with a defective ANP gene by homologous recombination; 2) Microinject ES cells with the mutant ANP gene into mouse blastocysts; 3) Transfer these blastocysts into the uteri of pseudopregnant mice; 4) Determine which of the resulting chimeric mice have the mutant ANP gene in the germ line; 5) Interbreed these chimeras to produce mice homo- and heterozygous for the ANP gene deletion. Physiologic consequences of ANP deletion will then be assessed. It is expected that the heterozygotes will be viable; the viability of the homozygotes is less certain. If not viable, the developing ANP-/ANP- embryos will be studied histologically to define the effects of ANP deletion. During Phase II of this grant, similar methods will be used to further characterize the physiology of ANP by utilizing homologous recombination to inactivate the ANP receptor genes. These studies will aid in the investigation of ANP function in normal and pathologic states. This may prove to be of particular import in helping to understand the dysregulation of blood pressure and volume homeostasis seen in liver failure, heart disease, renal disease, and hypertension.